1. Field of the Invention
This invention relates to the field of dye-sensitised solar cells and to a method for reducing the temperature necessary for sintering the metal oxide paste coating the electrode.
2. Description of the Related Art
Solar cells are traditionally prepared using solid state semiconductors. Cells are prepared by juxtaposing two doped crystals, one with a slightly negative charge, thus having additional free electrons (n-type semiconductor) and the other with a slightly positive charge, thus lacking free electrons (p-type semiconductor). When these two doped crystals are put into contact with each other, extra electrons from the n-type semiconductor flow through the n-p junction to reduce the lack of electrons in the p-type semiconductor. At the p-n junction, charge carriers are depleted on one side and accumulated on the other side thereby producing a potential barrier. When photons produced by sunlight strike the p-type semiconductor, they induce transfer of electrons bound in the low energy levels to the conduction band where they are free to move. A load is placed across the cell in order to transfer electrons, through an external circuit, from the p-type to the n-type semiconductor. The electrons then move spontaneously to the p-type material, back to the low energy level they had been extracted from by solar energy. This motion creates an electrical current.
Typical solar cell crystals are prepared from silicon because photons having frequencies in the visible light range have enough energy to take electrons across the band-gap between the low energy levels and the conduction band. One of the major drawbacks of these solar cells is that the most energetic photons in the violet or ultra-violet frequencies have more energy than necessary to move electrons across the band-gap, resulting in considerable waste of energy that is merely transformed into heat. Another important drawback is that the p-type layer must be sufficiently thick in order to have a chance to capture a photon, with the consequence that the freshly extracted electrons also have a chance to recombine with the created holes before reaching the p-n junction. The maximum reported efficiencies of the silicon-type solar cells are thus of 20 to 25% or lower for solar cell modules, due to losses in combining individual cells together.
Another important problem of the silicon-type solar cell is the cost in terms of monetary price and also in terms of embodied energy, that is the energy required to manufacture the devices. Dye-sensitised solar cells (DSSC) have been developed in 1991 by O'Regan and Gratzel (O'Regan B. and Gratzel M., in Nature, 1991, 353, 737-740). They are produced using low cost material and do not require complex equipment for their manufacture. They separate the two functions provided by silicon: the bulk of the semiconductor is used for charge transport and the photoelectrons originate from a separate photosensitive dye. The cells are sandwich structures as represented in FIG. 1 and are typically prepared by the steps of:                a) providing a transparent plate (1) typically prepared from glass;        b) coating this plate with a transparent conducting oxide (TCO) (2), preferably with doped tin oxide;        c) applying a paste of metal oxide (3), generally titanium dioxide, to the coated glass plate on the TCO side;        d) heating the plate to a temperature of about 450° C. to 600° C. for a period of time of at least one hour;        e) soaking the coated plate of step d) in a dye solution for a period of time of about 24 hours in order to covalently bind the dye to the surface of the titanium dioxide (4);        f) providing another TCO coated transparent plate further coated with platinum (5);        g) sealing the two glass plates and introducing an electrolyte solution (6) between said plates in order to encase the dyed metal oxide and electrolyte between the two conducting plates and to prevent the electrolyte from leaking.        
In these cells, photons strike the dye moving it to an excited state capable of injecting electrons into the conducting band of the titanium dioxide from where they diffuse to the anode. The electrons lost from the dye/TiO2 system are replaced by oxidising the iodide into triiodide at the counter electrode, whose reaction is sufficiently fast to enable the photochemical cycle to continue.
The DSSC generate a maximum voltage comparable to that of the silicon solar cells, of the order of 0.7 V. An important advantage of the DSSC, as compared to the silicon solar cells, is that they inject electrons in the titanium dioxide conduction band without creating electron vacancies nearby, thereby preventing quick electron/hole recombinations. They are therefore able to function in low light conditions where the electron/hole recombination becomes the dominant mechanism in the silicon solar cells. The present DSSC are however not very efficient in the lower part of the visible light frequency range in the red and infrared region, because these photons do not have enough energy to cross the titanium dioxide band-gap or to excite most traditional ruthenium bipyridyl dyes.
A major disadvantage of the DSSC resides in the high temperature necessary for sintering the metal oxide paste. Another drawback of the dye-sensitised solar cells lies in the long time necessary to dye the titanium dioxide nanoparticles: it takes between 12 and 24 hours to dye the layer of titanium dioxide necessary for solar cell applications. Another major difficulty with the DSSC is the electrolyte solution: the cells must be carefully sealed in order to prevent liquid electrolyte leakage and therefore cell deterioration.
There is thus a need to prepare robust solar cells that can be prepared rapidly at reduced cost.